1. Field of the Invention
The present invention relates to zoom lens systems and image pickup apparatuses equipped with such zoom lens systems. In particular, the present invention relates to a zoom lens system that is applicable to, for example, a video camera, an electronic still camera, or a silver-halide photographic camera.
2. Description of the Related Art
In recent years, image pickup apparatuses containing solid-state image pickup elements and used as video cameras, digital still cameras, broadcasting cameras, film cameras that use silver-halide films, etc., have become smaller and been given increased functionality. Accordingly, there are demands for compact, high-definition zoom lens systems with reduced overall length to be used as image-taking optical systems in such image pickup apparatuses.
As a zoom lens system that meets such demands, a so-called rear-focus zoom lens system is known, which performs focusing by moving lens units other than a first lens unit located proximate to the object side.
Generally, in comparison to a zoom lens system that performs focusing by moving the first lens unit, a rear-focus zoom lens system has a first lens unit with a smaller effective diameter, which easily allows for compactness of the entire lens system. Moreover, in a rear-focus zoom lens system, close-range shooting, or extra-close-range shooting in particular, can be readily performed. In addition, the use of small, lightweight lens units in a rear-focus zoom lens system reduces the driving force required for moving the lens units so as to allow for quick focusing.
A known example of a rear-focus zoom lens system is a four-unit zoom lens system that includes a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in that order from the object side towards the image side.
U.S. Pat. No. 5,963,378 and U.S. Pat. No. 6,166,864 disclose examples of a rear-focus four-unit zoom lens system that performs zooming by moving the second lens unit while performing focusing and correction of an image plane aberration occurring due to zooming by moving the fourth lens unit.
Other examples of a four-unit zoom lens system are known from Japanese Patent Laid-Open No. 08-050244 and U.S. Pat. No. 7,088,522. In these examples, when the zoom lens system performs zooming, the zoom lens system moves the lens units while also moving an aperture stop independently of the lens units, the aperture stop being disposed between the second lens unit and the third lens unit.
When the zoom lens system disclosed in Japanese Patent Laid-Open No. 08-050244 performs zooming from a wide-angle end to a telephoto end, the aperture stop is moved independently of the lens units along a locus convex towards the object side. For focusing, the zoom lens system moves the fourth lens unit. This zoom lens system has a zoom ratio of about 12.5.
On the other hand, the zoom lens system disclosed in U.S. Pat. No. 7,088,522 moves the aperture stop independently of the lens units towards the image side when performing zooming from a wide-angle end to a telephoto end, and has a zoom ratio of about 12 to 13.5.
In the case where the aperture stop is disposed near the third lens unit in the four-unit zoom lens systems described above, even if the zoom lens system is configured to distribute a sufficient quantity of light to the peripheral region of the screen, the light quantity decreases drastically from an 80% peripheral range of the screen. This decrease in the light quantity is conspicuous even under the same marginal illumination.
In contrast, U.S. Pat. No. 7,167,320 discloses a four-unit zoom lens system in which an aperture stop disposed on the object side of the third lens unit is moved independently of the third lens unit. In this four-unit zoom lens system, the aperture stop is moved by an appropriate distance so as to minimize a drastic fall-off in light quantity in the peripheral region of the screen when the zoom lens system is at the wide-angle end or at zoom positions near the wide-angle end.
In the zoom lens system according to U.S. Pat. No. 7,167,320, the first lens unit and the third lens unit are moved monotonously towards the object side when the zoom lens system performs zooming from the wide-angle end to the telephoto end. The aperture stop is configured to be moved independently of the lens units towards the object side. Accordingly, U.S. Pat. No. 7,167,320 discloses a zoom lens system with a zoom ratio of about 4.5 to 7, in which the off-axis performance is properly corrected at each zoom position.
Generally, in order to obtain a high zoom ratio in a zoom lens system while still achieving compactness thereof, the refractive powers of the lens units included in the zoom lens system may be increased and the number of lens elements may be reduced. However, a zoom lens system with such a configuration can be problematic in that the lens elements will have greater lens thicknesses in accordance with the increase in the refractive powers of the lens surfaces. For this reason, the length of the lens system cannot be sufficiently reduced, and at the same time, the zoom lens system will have difficulty in correcting the aberrations.
In addition, if a camera is to be given a collapsible function where the lens units can be collapsed and stored when the camera is not in use, increasing the refractive powers of the lens units will inevitably cause mechanical errors, such as tilting of the lens elements and the lens units.
In this case, if the sensitivity of the lens elements and the lens units is high, the optical performance can deteriorate and an image blur can occur during zooming. Therefore, in order to maintain high optical performance, it is desirable to minimize the sensitivity of the lens elements and the lens units.
In the aforementioned four-unit zoom lens systems, the light quantity tends to drop drastically in the peripheral region of the screen when the zoom lens system is at the wide-angle end or at zoom positions near the wide-angle end, thus lowering the image quality in the peripheral region of the screen.
Furthermore, in the aforementioned four-unit zoom lens systems, it is important to properly move the aperture stop during zooming to maintain a high image quality. If the movement condition of the aperture stop is not proper, it becomes significantly difficult to minimize the drastic fall-off of light quantity in the peripheral region of the screen.
To achieve high optical performance in the aforementioned four-unit zoom lens systems while obtaining a high zoom ratio and achieving compactness of the entire lens system, it is important to appropriately set the zoom type, the refractive powers of the lens units, and the movement conditions of the lens units and the aperture stop required for zooming.
Especially in the aforementioned four-unit zoom lens systems, it is extremely difficult to achieve high optical performance over the entire zoom range and to obtain a high zoom ratio at the same time if the refractive powers of the second lens unit and the third lens unit and the moving distances of the aperture stop and the third lens unit for zooming are not set properly.